3-CUBIC METER BIOGAS PLANT A CONSTRUCTION MANUAL
TABLE OF CONTENTS I. WHAT IT IS AND HOW IT IS USEFUL II. DECISION FACTORS Applications Advantages Disadvantages Considerations Cost Estimate III. MAKING THE DECISION AND FOLLOWING THROUGH IV. PRECONSTRUCTION CONSIDERATIONS By-Products of Digestion Location Size Heating and Insulating Digesters Materials Tools V. CONSTRUCTION Prepare Foundation and Walls Prepare the Gas Cap Drum Prepare Moisture Trap Prepare Mixing and Effluent Tanks VI. OPERATION Output and Pressure VII. VARIOUS APPLICATIONS OF BIOGAS AND DIGESTER BY-PRODUCTS Engines Fertilizer Improvised Stove Lighting VIII. MAINTENANCE Possible Troubles IX. TEST GAS LINE FOR LEAKS X. DICTIONARY OF TERMS
XI. CONVERSION TABLES XII. FURTHER INFORMATION RESOURCES A Listing of Recommended Resource Materials Useful Information for Methane Digester Designs APPENDIX I. DECISION MAKING WORKSHEET APPENDIX II. RECORD KEEPING WORKSHEET 3-CUBIC METER BIOGAS PLANT A CONSTRUCTION MANUAL I. WHAT IT IS AND HOW IT IS USEFUL Biofuels are renewable energy sources from living organisms. All biofuels are ultimately derived from plants, which use the sun's energy by converting it to chemical energy through photosynthesis. When organic matter decays, burns, or is eaten, this chemical energy is passed into the rest of the living world. In this sense, therefore, all life forms and their by-products and wastes are storehouses of solar energy ready to be converted into other usable forms of energy. The kinds and forms of the by-products of the decay of organic matter depend on the conditions under which decay takes place. Decay (or decomposition) can be aerobic (with oxygen) or anaerobic (without oxygen). An example of anaerobic decomposition is the decay of organic matter under water in certain conditions in swamps. Aerobic decomposition yields such gases as hydrogen and ammonia. Anaerobic decomposition yields primarily methane gas and hydrogen sulfide. Both processes produce a certain amount of heat and both leave a solid residue that is useful for enriching the soil. People can take advantage of the decay processes to provide themselves with fertilizer and fuel. Composting is one way to use the aerobic decay process to produce fertilizer. And a methane digester or generator uses the anaerobic decay process to produce both fertilizer and fuel. One difference between the fertilizers produced by these two methods is the availability of nitrogen. Nitrogen is an element that is essential to plant growth. As valuable as compost is, much of the nitrogen held in the original organic materials is
lost to the air in the form of ammonia gas or dissolved in surface runoff in the form of nitrates. The nitrogen is thus not available to the plants. In anaerobic decomposition the nitrogen is converted to ammonium ions. When the effluent (the solid residue of decomposition) is used as fertilizer, these ions affix themselves readily to soil particles. Thus more nitrogen is available to plants. The combination of gases produced by anaerobic decomposition is often known as biogas. The principle component of biogas is methane, a colorless and odorless gas that burns very easily. When handled properly, biogas is an excellent fueld for cooking, lighting, and heating. A biogas digester is the apparatus used to control anaerobic decomposition. In general, it consists of a sealed tank or pit that holds the organic material, and some means to collect the gases that are produced. Many different shapes and styles of biogas plants have been experimented with: horizontal, vertical, cylindrical, cubic, and dome shaped. One design that has won much popularity, for reliable performance in many different countries is presented here. It is the Indian cylindrical pit design. In 1979 there were 50,000 such plants in use in India alone, 25,000 in Korea, and many more in Japan, the Philippines, Pakistan, Africa, and Latin America. There are two basic parts to the design: a tank that holds the slurry (a mixture of manure and water); and a gas cap or drum on the tank to capture the gas released from the slurry. To get these parts to do their jobs, of course, requires provision for mixing the slurry, piping off the gas, drying the effluent, etc. In addition to the production of fuel and fertilizer, a digester becomes the receptacle for animal, human, and organic wastes. This removes from the environment possible breeding grounds for rodents, insects, and toxic bacteria, thereby producing a healthier environment in which to live. II. DECISION FACTORS Applications: * Gas can be used for heating, lighting, and cooking. * Gas can be used to run internal combustion engines with modifications. * Effluent can be used for fertilizer.
Advantages: * Simple to build and operate. * Virtually no maintenance--25-year digester lifespan. * Design can be enlarged for community needs. * Continuous feeding. * Provides a sanitary means for the treatment of organic wastes. Disadvantages: * Produces only enough gas for a family of six. * Depends upon steady source of manure to fuel the digester on a daily basis. * Methane can be dangerous. Safety precautions should be observed. CONSIDERATIONS Construction time and labor resources required to complete this project will vary depending on several factors. The most important consideration is the availability of people interested in doing this project. The project may in many circumstances be a secondary or after-work project. This will of course increase the length of time needed to complete the project. The construction times given here are at best an estimation based on limited field experience. Skill divisions are given because some aspects of the project require someone with experience in metalworking and/or welding. Make sure adequate facilities are available before construction begins. The amount of worker-hours needed is as follows: * Skilled labor - 8 hours * Unskilled labor - 80 hours * Welding - 12 hours Several other considerations are: * The gas plant will produce 4.3 cubic meters of gas per day on the daily input from eight cattle and six humans.
* The fermentation tank will have to hold approximately 7 cubic meters in a 1.5 X 3.4 meters deep cylinder. * A gas cap to cover the tank should be 1.4 meters in diameter X 1.5 meters tall. COST ESTIMATE $145-800 (U.S., 1979) includes materials and labor. ___________ (*)Cost estimates serve only as a guide and will vary from country to country. III. MAKING THE DECISION AND FOLLOWING THROUGH When determining whether a project is worth the time, effort, and expense involved, consider social, cultural, and environmental factors as well as economic ones. What is the purpose of the effort? Who will benefit most? What will the consequences be if the effort is successful? And if it fails? Having made an informed technology choice, it is important to keep good records. It is helpful from the beginning to keep data on needs, site selection, resource availability, construction progress, labor and materials costs, test findings, etc. The information may prove an important reference if existing plans and methods need to be altered. It can be helpful in pinpointing "what went wrong?" And, of course, it is important to share data with other people. The technologies presented in this series have been tested carefully, and are actually used in many parts of the world. However, extensive and controlled field tests have not been conducted for many of them, even some of the most common ones. Even though we know that these technologies work well in some situations, it is important to gather specific information on why they perform better in one place than in another. Well documented models of field activities provide important information for the development worker. It is obviously important for a development worker in Colombia to have the technical design for a plant built and used in Senegal. But it is even more important to have a full narrative about the plant that provides details on materials, labor, design changes, and so forth. This model can provide a useful frame of reference. A reliable bank of such field information is now growing. It exists to help spread the word about these and other technologies, lessening the dependence of the developing world on
expensive and finite energy resources. A practical record keeping format may be found in Appendix II. IV. PRECONSTRUCTION CONSIDERATIONS The design presented here is most useful for temperate or tcm1x9.gif (600x600)
tropical climates. It is a 3-cubic meter plant that requires the equivalent of t